In association with expansion of general-purpose usage of the electrophotographic technique, an image forming apparatus employing an electrophotographic system is being used not only in office applications but also in the industrial printing field and light printing field, where an offset printing has been conventionally the mainstream. Also, along with an increasing demand for stable and mass printing of an image requiring high image quality, such as photograph, it is more strongly demanded for an electrophotographic photoreceptor (hereinafter, sometimes referred to as “photoreceptor”) as a core of the electrophotography process, for example, to reduce the environmental change such as moisture or the image abnormality such as image memory, improve the abrasion resistance, and stabilize the electrostatic potential.
In order to meet these demands for the photoreceptor, various improvements have been made on the photoreceptor composition. With respect to the stability against environmental change, use of less humidity-dependent gallium phthalocyanine in place of conventionally employed titanyl phthalocyanine has been proposed (Patent Documents 1 and 2). Also, with respect to stabilization of the electrostatic potential, a charge transport material having a specific structure, among others, a triarylamine-based compound having a fluorenyl group, has been proposed (Patent Document 3). In addition, use of a polyester resin, among others, a polyarylate resin that is a generic term for a full aromatic polyester resin, in place of the conventionally employed polycarbonate resin has been proposed so as to, for example, improve abrasion resistance, improve an image defect such as filming, or improve toner transferability (Patent Document 4).
Out of image memories, as for the memory attributable to the effect of transfer load, in a reverse development system, the charge voltage and the transfer voltage are opposite in polarity and therefore, a so-called transfer memory, that is, a phenomenon where the chargeability becomes different by the effect of transfer, may be produced, giving rise to a defect such as density unevenness on image. With respect to reduction of the transfer memory, it is disclosed that a combination of specific charge transport materials works effectively (Patent Document 5).
Incidentally, the full color image forming method includes mainly a tandem system and a four-cycle system, and the transfer system on a printing medium includes, for example, a direct transfer system, a transfer drum system, an intermediate transfer system, and a multiple development-batch transfer system. Among these, a tandem system, that is, a color image forming apparatus where respective color images are formed by independent image-forming units and sequentially transferred, is an excellent image forming method, because many kinds of recording materials are usable, the full-color quality is high, and a full-color image can be obtained at a high speed.
In the case of a tandem system, high speed printing is available, but on the other hand, a system of forming respective color images by a plurality of image forming units and sequentially transferring the images is employed. Therefore, in the tandem system, the toner image transferred on a transfer medium (an intermediate transfer medium or a recording material) becomes thicker as it progresses toward the later image forming unit, and a larger transfer voltage is applied in many cases to transfer the toner layer formed on the electrophotographic photoreceptor. This brings about a tendency that charge injection into the photosensitive layer upon loading of the above-described opposite polarities is more encouraged and a clearer density difference is produced on the image depending on the site, as a result, a so-called transfer memory is liable to occur.